1. Field of the Invention
The present invention is intended to provide a lens for use in a lamp which can provide an even brightness distribution and a good visibility, and a method of producing a die for forming that lens.
2. Description of the Background Art
Among vehicular lamps is a type having a structure in which an inner lens for controlling direct light from a light source and reflection light from a reflector is disposed in a lamp space defined by a lamp body and an outer lens. The inner lens is a formed product of a transparent synthetic resin, and has Fresnel lens steps and prism steps on its one surface.
As shown in FIG. 16, a lamp 1 is designed to have a curved exit surface of an outer lens 2 that conforms to a body shape of a vehicle.
In the lamp 1, the optical axis x--x of a reflector 3 extends in the front-rear direction of the vehicle passing through the center of a filament of a bulb 4. An inner lens 5 is disposed between the bulb 4 and the outer lens 2. That is, the inner lens 5 is placed immediately inside the outer lens 2, and is influenced, like the outer lens 2, by the vehicle body shape to have an exit surface 6 assuming a curved shape.
The inner lens 5 consists of a plate-like portion 5a and a curved portion 5b that is continuous with the plate-like portion 5a and curved increasingly as the position reaches one end in the longitudinal direction. Fresnel lens steps 7, 7, . . . are formed on the inner surface in the vicinity of the optical axis x--x, and prism steps 8, 8, . . . are formed around the Fresnel lens steps 7, 7, . . .
FIG. 17 is a sectional view showing the main part a of a plate-like inner lens 5.
Fresnel lens steps b, b, . . . are formed on an incident surface of the inner lens a in the vicinity of the optical axis x--x of the reflector 3, and prism steps c, c, . . . are formed around the Fresnel steps b, b, . . . Through the refraction by the Fresnel lens steps b, b, . . . , the paraxial rays of the light emitted from a bulb d are controlled to become in parallel with the optical axis of the lamp. The outer rays of the light from the bulb d that depart from the optical axis of the lamp to go toward the peripheral area of the inner lens a are controlled through the total reflection by the prism steps c, c, . . . to become in parallel with the optical axis.
This structure is employed because the paraxial rays have small incident angles with respect to the inner lens a and can be controlled through the refraction phenomenon, but the outer rays departing from the optical axis have large incident angles with respect to the inner lens a. Accordingly, it is difficult to control the outer rays through refraction.
In order to accommodate the recent design trend that vehicle bodies are rounded or streamlined to improve the aerodynamic characteristics of vehicles and to satisfy requirements on design, it is necessary to design a lamp shape to have a curve that conforms to the external shape of a vehicle body or to have an inclination to the vertical direction. Therefore, it is not possible for the inner lens to be limited to a plate-like shape, that is, in general the inner lens is required to include a curved shape.
FIG. 18 conceptually shows an example of a method of forming lens steps on a curved surface of an inner lens.
To simplify the description, it is assumed that lens steps are to be formed on a spherical surface, as shown in FIG. 18. There may be conceived a method in which a plate-like inner lens f on which lens steps are to be formed based on concentric reference circles e, e, . . . is employed as a reference model of design, and the concentric reference circles e, e, . . . are projected onto a spherical surface g. In this case, Fresnel lens steps and prism steps are formed on the spherical surface g based on reference circles h, h, . . . that are concentric to the optical axis.
While the above method permits a relatively easy design, it will encounter a difficulty in precisely controlling the light paths. As a result, parallel rays cannot be obtained over the entire surface of the inner lens, and the brightness distribution will be uneven.
This is a natural result of a fact that fine optical designing is not performed on the lens steps in accordance with the surface shape of the inner lens. The portion of the inner lens that is not very curved, i.e., generally flat portion 5a, will not cause any problems. But the portion 5b in which the curvature varies greatly will cause a considerable deviation from the desired brightness distribution due to a contribution of unexpected rays.
To avoid the above problem, it is necessary to alter the method of forming the lens steps. However, according to the above method, a proper course of designing cannot be obtained easily. Therefore, much time and work are needed to design the inner lens, and its final design and performance will depend on experiences of a designer.